Several methods have been sought to produce high gloss bottles presenting good processability and good mechanical properties but all the blends and techniques used so far present various disadvantages.
High gloss high density polyethylene (HDPE) has been used: it is characterised by a very narrow molecular weight distribution that is typically inferior to 8. The molecular weight distribution can be completely defined by means of a curve obtained by gel permeation chromatography. Generally, the molecular weight distribution (MWD) is more simply defined by a parameter, known as the dispersion index D, which is the ratio between the average molecular weight by weight (Mw) and the average molecular weight by number (Mn). The dispersion index constitutes a measure of the width of the molecular weight distribution. It is known that a resin of narrow molecular weight distribution will produce plastic containers of very high gloss but simultaneously, that such resin will be very difficult to process and will be characterised by very poor mechanical properties. It has also been observed that said resins have poor mechanical properties, particularly, a very low environmental stress crack resistance (Modern Plastic International, August 1993, p. 45).
The coextrusion of high density polyethylene (HDPE) with a thin external layer of polyamide has been used to produce bottles of very high gloss but that method suffers the major drawback of necessitating an adhesive layer between the HDPE and the polyamide layers.
The coextrusion of high density polyethylene and an external layer of low density polyethylene leads to bottles with a fair gloss. These bottles however have an unpleasant greasy touch and offer a very poor resistance to scratching.
In another method, disclosed in co-pending European Patent Application no 00201155.9, high gloss plastic containers comprise an internal layer including a polyolefin and an external layer including a styrenic component containing from 40 to 85 wt % of styrene, based on the weight of the external.
There is thus a need for a method for efficiently producing hollow packagings of very high gloss as well as good processability and mechanical properties by injection moulding.
An aim of the present invention is to produce hollow packagings that offer simultaneously the desired glossy appearance and a good resistance to scratching.
It is also an aim of the present invention to obtain glossy hollow packagings with good processability and good mechanical properties.